debug: add /debug to display key debug info

I deployed this on testing2: Check out the found page of my key. It contains a link in the top right that's only visible on hover to https://testing2.keys.openpgp.org/debug?q=D4AB192964F76A7F8F8A9B357BD18320DEADFA11

I imagine this could be useful for developers to have, to have a quick thing to point to, e.g. when something is wrong with a key. It's similar to what was asked for in #90 (closed)

@nwalfield @teythoon @dkg Thoughts on whether this is actually useful? Or just bloat? Or even detrimental to users or security, in some way?